In the design of aerodynamic or streamlined fluid dynamic components such as airfoil panels utilized as wing surfaces, tail surfaces, propellers, ship stabilizers or other devices which move through a fluid medium, it is desirable sometimes to impart various amounts of spanwise twist or pitch to the components as a control function. A classic example of twisting or pitching a panel to obtain a control function may be seen in the wing warping technique utilized by the Wright Brothers and other aviation pioneers to obtain lateral control of their aircraft. Such early applications of wing twisting involved wings which had thin, cambered airfoil sections. These structures inherently had low torsional resistance and could be deformed easily when subjected to the application of a torsional force without sacrificing strength in bending.
As the development of airfoil panels has unfolded, the panels have become relatively thick and have two surfaces resulting in a substantially tubular shape in order to increase their efficiency. Additionally, the thickness of the panel walls has increased substantially. As a result, the panels are torsionally rigid. Consequently, when a modern airfoil panel is twisted about a spanwise axis through the application of a torsional force, the entire panel tends to pitch as a unit. In other words, the panel is pitched uniformly along its span. In contrast thereto, if the panel could flex in torsion the application of a torsional force to the panel would result in the panel pitching with a spanwise gradient. In this case the panel would be twisted by different amounts along its span. The ability to pitch a modern relatively thick airfoil panel with a spanwise gradient becomes advantageous in many instances. For example, where the panel is utilized as a flapping wing in an ornithopter, pitching the panel over a gradient from root to tip corresponds with the velocity vector of the fluid which flows over the panel, which is graded linearly from root to tip and increases the efficiency of the device.
In addition to the ability to pitch a modern two surface panel with a spanwise gradient when the panel has been subjected to a torsional force, it has been found to be desirable to be able to mechanically pitch the panel so as to impart a spanwise gradient independent of the application of a fluid- dynamic torsional force to the panel.